In 1877, the French astronomer Edouard Stephan discovered the first galaxy cluster beyond our own local group. Being a difficult target for small telescopes, this cluster of five galaxies in Pegasus is an astronomer's treat.
Four of the five galaxies you see at the centre of my sketch actually have a physical relationship (catalogued as Hickson Compact Group 92 or Arp 319) and given their closeness and relative speeds they will soon merge with each other. The collision speed of NGC7318b (Cfr. the bottom image with labels), for instance, is so mind-bogglingly huge - we're talking about MILLIONS of km/h! - that it generates a shock wave, similar to sonic boom but travelling through intergalactic gas, that is larger than our Milky Way! The gas is heated up to millions of degrees Centigrade and starts to emit X-rays, so powerful that we can detect them here on Earth, in spite of the 210 to 340 million light-year distance.
NGC7320, on the other hand, appears to move in a completely different direction and at a much slower speed compared to us. From this we conclude that this galaxy is no part of the group at all and that it is in fact much closer to us (about 39 million light-years).
Extremely faint and somewhat distant NGC7320c, however, seems to share the same motion of the four group members and in fact a trail of gas and dust has been detected that leads from NGC7319 in its direction. Therefore it is more than likely also a member.
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